a) Field of the Invention
The present invention relates to a real image mode variable magnification finder optical system for use in photographic cameras, video cameras or the like.
b) Description of the Prior Art
Real image finder optical systems are well known as finder optical systems formed independently of photographing optical systems. The real image finder optical system can obtain a clear view of a field frame and a good image within the field frame. Recently, however, it is required to provide the finder with a smaller size and higher ratio of magnification change. In the real image finder optical system, an image erecting system is needed to convert an inverted image (an intermediate image) formed by an objective system into an erect image. In the case in which this image erecting system is constituted by a reflecting member such as a Porro prism or the like, it is known that the total length of the finder optical system can be made shorter by positioning an intermediate imaging position within the image erecting system. That is, generally, if one of four reflecting surfaces conventionally included in an eyepiece system is incorporated in an area of a back focus, the total length of the finder optical system will be able to be shortened. Specifically, if two or more reflecting surfaces are incorporated in the objective system, it is known that the optical performance of the eyepiece system can be well maintained, because the distance between the intermediate imaging position and the eyepiece system can be shortened. Accordingly, an objective system which can assure a long back focus has been in recent demand. For example, objective systems comprising a first lens unit with a negative refracting power, a second lens unit with a positive refracting power and a third lens unit with a negative refracting power are disclosed in Japanese Patent Preliminary Publication Nos. Hei 3-4217 and Hei 5-53054, respectively. These are conventionally known objective systems, each having a long back focus and by which a variable magnification ratio of the order of three times is obtained.
Japanese Patent Preliminary Publication No. Hei 3-4217 discloses a finder optical system also including a magnification changing operation mechanism, which is made small by moving only a second and third lens units each having a relatively small lens diameter. As shown in FIGS. 1A, 1B and 1C, this optical system is composed of a first lens unit G.sub.1, a second lens unit G.sub.2 and a third lens unit G.sub.3. When the magnification is made larger, the second lens unit G.sub.2 is shifted to the object side (the left side in the drawings) along the optical axis and a change of diopter by this change of magnification level is corrected by shifting the third lens unit G.sub.3 along the optical axis. In this system, the third lens unit G.sub.3 is adapted to be shifted to the object side when magnification of the finder optical system is changed from a wide position to a middle position thereof and is adapted to be moved to the eyepiece system side (the right side in the drawings) when magnification is to be changed from the middle position to a telephoto position thereof. The total length L.sub.0 of the objective system is constant.
Further, in the optical system disclosed in Japanese Patent Preliminary Publication No. Hei 5-53054, a first lens unit G.sub.1 and a second lens unit G.sub.2 are adapted to be moved respectively along the optical axis when magnification of the finder optical system is to be changed between a wide position, a middle position and a telephoto position thereof, as shown in FIGS. 2A, 2B and 2C. This finder optical system is intended to provide a telephoto type optical system which is shortened in its total length by using the above-mentioned two lens units for varying magnification having a positive refracting power in combination and a third lens unit G.sub.3 having a negative refracting power. Variation of magnification for the telephoto position is performed by shifting the second lens unit G.sub.2 toward the object side (the left side in the drawings), and a change of diopter by the variation of magnification is corrected by moving the first lens unit G.sub.1. The first lens unit G.sub.1 is shifted toward the eyepiece system side along the optical axis when magnification is to be changed from the wide position to the middle position thereof and is shifted toward the object side along the optical axis when magnification is to be changed from the middle position to the telephoto position thereof. Accordingly, the total length of the finder optical system becomes shortest at the middle position of the variation of magnification and becomes equal approximately at the wide position and the telephoto position thereof. Thus, a small sized objective system as a whole can be provided.
However, the optical system disclosed in Japanese Patent Preliminary Publication No. Hei 3-4217 has relatively high manufacturing cost and is large because it comprises many lens elements and the objective system portion occupies, in the optical system, considerable space. On the other hand, the optical system disclosed in Japanese Patent Preliminary Publication No. Hei 5-53054 can be made such that it is small. However, if one tries to obtain a higher magnification ratio change in this small optical system, a large amount of variation in aberrations of the optical system will occur when magnification of the optical system is changed, and, as a result, one will not obtain a good image of the field of view.
Further, examples of finder optical systems that include variable magnification objective systems consisting of a first lens unit with a negative refracting power, a second lens unit with a positive refracting power and a third lens unit with a negative refracting power are disclosed in Japanese Patent Preliminary Publication Nos. Hei 3-233420, Hei 4-230719 and Hei 5-346610, respectively. As each of these objective systems is so formed as to have a short back focus, two or more reflecting members can not be arranged in the area having the back focus.
Furthermore, at the telephoto position of the finder optical system, in which a field angle thereof becomes narrow, flare may occur by injurious light rays which enter into the finder optical system from outside of the field of view. As is well known, it is preferable to arrange a light interrupting member before the objective system in order to obtain a good image of visual field without flare. However, in the prior finder optical systems as described above, as the total length of the finder optical system is approximately the same at the wide position (FIG. 3A) and the telephoto position (FIG. 3B) thereof as shown in FIGS. 3A and 3B, if the light interrupting member 20 is disposed to fit the finder optical system in the telephoto position, a part of the visual field will be cut off when the finder optical system is adjusted to the wide position. To the contrary, if the light interrupting member 20 is disposed to fit the finder optical system in the wide position, a good image of visual field will be spoiled due to insufficient removal of injurious light rays when the finder optical system is adjusted to the telephoto position.